The present invention relates to a reception method in a system employing a code division multiple access method, in which a received transmission is digitized, the received transmission comprising a desired signal and a plurality of interfering signals, which signals are each multiplied by a code of a known plurality of codes and in which method at least some of the signals interfering with the desired signal are cancelled and the desired signal is detected after the interference cancellation.
The present invention can specifically be applied to CDMA data communication systems. CDMA is a multiple access method based on a spread spectrum technique, and it has been recently put into use in cellular radio systems in addition to previously used FDMA and TDMA. CDMA has many advantages over the prior methods, such as simplicity of frequency planning and spectrum efficiency.
In the CDMA method, the narrowband data signal of the user is multiplied to a relatively wide band by a spreading code having a considerably broader band than the data signal. In known test systems, bandwidths such as 1.25 MHz, 10 MHz and 25 MHz have been used. In connection with multiplying, the data signal spreads to the entire band to be used. All users transmit by using the same frequency band simultaneously. A separate spreading code is used over each connection between a base station and a mobile station, and the signals of different users can be distinguished from one another in the receivers on the basis of the spreading code of each user. The intention is to select the spreading codes in such a way that they are mutually orthogonal, i.e. they do not correlate with one another.
Correlators in conventionally implemented CDMA receivers are synchronized with a desired signal which they recognize on the basis of the spreading code. In the receiver the data signal is restored to the original band by multiplying it by the same spreading code as in the transmission step. Ideally, the signals that have been multiplied by some other spreading code do not correlate and are not restored to the narrow band. In view of the desired signal, they thus appear as noise. The object is to detect the signal of a desired user from among a number of interfering signals. In practice, the spreading codes correlate, and the signals of other users make the detection of the desired signal more difficult by distorting the received signal linearly. This interference caused by the users to one another is called multiple access interference.
Several reception methods have been developed to eliminate the deterioration of the signal quality caused by the multiple access interference. These comprise both a conventional one user reception and methods allowing a simultaneous multiuser detection. In a conventional one user reception the received transmission is correlated with a linear, matched filter which ignores all other signals in the transmission than that of a desired user. This is simple to implement, but it is susceptible to a so called near-far phenomenon, in which a transmitter close to a base station may jam the signals of transmitters located farther away.
An optimal multiuser detector (MUD) comprises a plurality of linear, matched filters and a Viterbi-detector. One known linear multiuser detector is a LS detector (least squares detector) which is called a decorrelating detector. This detector requires mutual cross-correlations of codes used. Also other linear detectors have been developed, based on a criterion of the minimum mean square error (MMSE). These methods are also akin to subspace methods, the aim of which is to estimate the signal subspace of a received signal. A drawback of these methods is the complexity which grows exponentially with the number of users.
The purpose of the present invention is to provide a reception method with which the drawbacks of known methods can be eliminated. The purpose of the invention is to provide a reception method and a receiver that is not too complicated to implement and that cancels efficiently the effect of multiuser interference from a desired signal.
This is achieved with a method of the type described in the preamble, the method being characterized in that the number of interfering signals and the codes used by them are estimated in a received transmission, and the effect of perceived signals are cancelled from a desired signal.
The invention also relates to a receiver in a system employing a code division multiple access method, the receiver comprising means for digitizing a received transmission comprising a desired signal and a plurality of interfering signals, each multiplied by a code of a known code set, and which receiver comprises means for cancelling at least some of the interfering signals from the desired signal and means for detecting the desired signal after the interference cancellation. The receiver in accordance with the invention is characterized in that it comprises means for estimating the number of interfering signals and the codes used by them, in a received transmission, and means for cancelling the effect of perceived signals from the desired signal.
Several advantages are achieved with the method of the invention. To function, the method of the invention only needs information on code set used. For instance, information on the codes of active users or the number of users need not to be signalled to the receiver. Thus the signalling load of the system does not increase. The method of the invention is not susceptible to errors either. In case the number of errors grows in estimation for some reason or other, the quality of the signal does not change suddenly, but the quality deteriorates gradually.
The codes and number of active users need not necessarily be estimated in connection with every symbol detection either, since these parameters change slowly as compared with the symbol speed. As a consequence the processing capacity required can be reduced, if desired. Thus the method can be applied with a desired accuracy Depending on the processing capacity of a receiver concerned.